1. Field of the Invention
The present invention relates to a lens barrel suitable for use in a small camera such as a digital camera or the like and a camera using the lens barrel, particularly, to an improvement in a cam shape to move lenses in a collapsible lens barrel.
2. Description of the Prior Art
In recent, a miniaturization of a lens barrel has been advanced rapidly in response to a downsize of a camera, especially, a digital camera. The digital camera is adapted to image a subject by a solid-state image sensing device, such as a CCD (charge-coupled device), obtain image data of a still image or moving image of the subject and then to store digitally in an electronic recording medium. The digital camera, also, can be miniaturized easily, because a taking lens becomes small, if a small solid-state image sensing device is used.
By the way, because a current consumption in such a digital camera is large in structure, a used possible time tends to be short and therefore saving of current consumption is requested. In particular, a load becomes large in driving the lens barrel and therefore there is raised a problem that the current consumption becomes large.
As a small lens barrel, there is used a structure of one and two step collapsible lens barrels and so on, which are collapsed by a cam when photographing. In the structure, the rotation of a cam follower engaging with a cam on a cam cylinder is limited by a groove for direct advance to go straight a lens group and then the lens group combined with the cam follower is forwardly and backwardly moved by the rotation of the cam cylinder. There is, also, well known a collapsible lens barrel of one step type.
The one step type collapsible lens barrel comprises a fixing cylinder which is fixed to a substrate and which has a direct advance groove for carrying out a rotating limitation, a cam cylinder which is rotatably arranged peripherally of the fixing cylinder and which has a cam groove, and a lens frame which has a cam follower engaging with the cam groove and direct advance groove and which holds the lens group. By the rotation of the cam cylinder, the lens group is moved into a any position along direct advance groove.
The two step type collapsible lens barrel is composed of a fixing cylinder which is fixed to a substrate and which has a cam groove, a rotating cylinder having a cam follower engaging with the cam groove of the fixing cylinder, a direct advance cylinder which is disposed within the rotating cylinder so as to move forwardly and backwardly while the rotation being limited together with the rotating cylinder and which has a direct advance groove, lens frames which each have cam followers engaging with the cam groove of the rotating cylinder and the direct advance groove of the direct advance cylinder and which hold the lens groups. An inner surface of the rotating cylinder is formed with a cam groove for moving the lens frames.
By the rotation of the rotating cylinder, the direct advance cylinder is moved forwardly and backwardly along the cam groove of the fixing cylinder while rotating and then the lens frames engaging with the cam groove of the rotating cylinder are moved into any positions in forward and backward directions, with the rotation being limited by the direct advance groove of the direct advance cylinder.
In the lens barrel, the lens groups must be driven accurately in character and a space between the cam groove and cam follower must be substantially 0 (zero). However, working the cam groove in such a manner that the space becomes zero is difficult, an evenness of a torque necessary to rotate the rotating cylinder is generated by variation in a width of the cam groove. In order to resolve this problem, a play is provided between the cam groove and cam follower and a high accurate cam surface having a high shape accuracy is formed on one of the opposite walls of the cam groove and then the cam follower is contacted under a pressure with and moved sidably on the high accurate cam surface by biasing the cam follower with a resilient member such as a spring or the like. With the construction, it is possible to hold an accurate lens positional relationship.
In other words, the lens frames are biased always in a reverse direction by disposing a spring between the lens frames, or the rotating cylinder is biased always forwardly by disposing a pressure spring between the rotating cylinder and substrate in the two step type collapsible lens barrel, thus to carry out accurately the movement of the lens groups along the cam shape.
The collapse in the collapsible lens barrel can be carried out in such a manner that the lens groups are approached as soon as possible, as in a coaxial telescopic cylinder in order to miniaturize the whole in case of non-photographing.
Conventionally, the cam for moving the lens groups has usually a straight line having a constant angle to an imaged plane in a collapsed region from a photographing state to a collapsed state to carry out the collapse of the lens groups and rotating cylinder. If an amount of movement of the lens groups is large between the collapsed state and photographing state, it is necessary to take a large cam angle and therefore a very large energy is required for a collapsed operation. Further, the lens groups are approached in the collapsed state and the spring for biasing must be contracted, accordingly.
A large energy is required to contract the spring, and a necessary energy becomes large in proportion to the contracted amount of the spring as approaching to the lens groups. The combined energy is required to impart to a rotating torque of the cam and becomes the maximum just before completion of collapse.
Usually, it is considered that a DC (direct current) motor or stepping motor is used as a driving source for imparting a rotational torque. In case of the DC motor, there is a problem that a current value becomes large in order to generate a large torque. In the stepping motor, it is necessary to select a large motor or use a small resistance. As the resistance is lowered in the stepping motor, of course, there is a problem that the current value is elevated.
In order to resolve these problems, there is known a conventional lens barrel attempting to minimize an energy necessary to collapse with a less friction by widening a width of the cam in collapsed region than that as in the photographing region to form a play between the cam and cam follower, as disclosed in Japanese Patent Laid-Open No. 2002-162552.
However, the structure disclosed in Japanese Patent Laid-Open No. 2002-162552 is not effective to a lens barrel desiring to maintain accurately a position of lens groups by bias of a spring.
In a lens barrel in which a load is added to close a barrier when lens groups are collapsed, a combined energy is eliminated by minimizing an angle of a cam in a loaded portion and thus by minimizing an energy by the angle of the cam.
However, in the lens barrel, energy on a portion closing the barrier is merely eliminated, in a collapsed region just before the barrier is closed, a great energy is required in order to have the large angle of the cam and therefore it is not make any difference from the conventional lens barrels, as described above.